The present invention is directed to circuitry for a switch which selects one of two or more mobile telephone antennas which has the best signal, and, more particularly, to such a circuit which operates without having to make any alterations in the mobile telephone itself.
In certain cases it is advantageous for a radio telephone to select one of several antennas, each antenna having different reception and transmission characteristics. The best antenna at each given time can be selected according to the signals being received by the radio telephone and is switched (either manually or automatically) to the radio part of the mobile telephone. This principle is called antenna diversity.
In the committee preparing the system specifications for the dual mode radio telephone system which will be adopted in the U.S.A., a proposal has been made to make mobile station receiver diversity optional, e.g., the use of a diversity antenna would not be compulsory. One advantage of antenna diversity is that a significant improvement in transfer quality when data which mediates a service is being transmitted from a support station to a mobile station (for example, at a rate of 9.6 kb/s) because an antenna intended for the reception of this data signal is used, rather than the same antenna for mediating both speech and data.
For example, prior art car phones using a diversity antenna comprise the mobile telephone itself, two antenna, and a switch between the telephone and the antenna. This switch allows the mobile telephone to be switched to either one of the antenna. The switch is a controllable switch in the mobile telephone and receives its control from the mobile telephone logic via a separate control line. A control signal from the control line cause the antenna to be switched according to the best signal obtained. Field intensity, for example, is a suitable criterion on which to base this selection. In a digital time division multiplex system (TDMA), for example, it is advantageous to make the selection before the time interval for reception, since antenna switching carried out within that time interval causes error bursts in the bit queue received.